1. Field of the Invention
The present invention relates to a process for producing pitch-based carbon fibers. More particularly, it pertains to a process for efficiently producing pitch-based carbon fibers by liquid-phase oxidative polymerization of pitch fibers to easily infusibilize the fibers at a low temperature in a short time.
2. Description of Related Art
Pitch-based carbon fibers have heretofore been produced by firstly spinning a raw material pitch to form pitch fibers and then subjecting the resultant pitch fibers to oxidative polymerization in gaseous phase of air at an elevated reaction temperature of 160.degree. to 180.degree. C. or higher to form infusibilized fibers. With respect to the aforestated production process, however, in the case where there was used as raw material, a pitch having a softening point lower than the reaction temperature, especially an optically isotropic pitch, it was hardly possible to perform infusibilization treatment by reason of the high reaction temperature.
In the case of producing optically isotropic pitch-based carbon fibers, there is employed a raw material pitch comprising optically isotropic pitch as the principal component. The optically isotropic pitch, however, has involved the problem that because of its low softening point as well as a high contents of low molecular components, the infusibilizing treatment of the pitch fibers obtained by spinning the pitch causes the pitch fibers to melt during the treatment making it extremely difficult to preserve the original fibrous shape and thus to produce infusibilized fibers, and even if it is possible to produce infusibilized fibers, a long time is required.
As described hereinbefore, it is the present status of the conventional process for producing infusibilized fibers by gas-phase oxidative polymerization that the use of optically isotropic pitch as the raw material pitch makes it difficult to effect infusibilization at a high reaction temperature on account of the low softening point of the raw material pitch itself.
Aside from the foregoing, infusibilized fibers have heretofore been obtained by spinning mesophase pitch to form mesophase pitch fibers and subsequently subjecting the resultant fibers to oxidative polymerization in the air at a high temperature of about 200.degree. to 400.degree. C. However, since the above-mentioned process allows oxygen to penetrate to the inside of the fibers and decreases the orientation properties of the pitch molecules owing to the oxidation, growth of crystal is impaired in the later carbonization step and structural defect is brought about by the release of the introduced oxygen, thus causing difficulty in achieving the carbon fibers with high performance in physical properties.
In view of the above circumstances facing such difficulty, intensive research and investigation were concentrated by the present inventors into the development of a process enabling infusibilization of the pitch fibers even at a low reaction temperature by cancelling the disadvantages inherent to the conventional process.
As a result, it has been found that infusibilizing treatment is facilitated by subjecting the pitch fibers to liquid-phase oxidative polymerization under specific reaction conditions adjusted to a relatively low temperature to proceed with infusibilization. The present invention has been accomplished on the basis of the aforestated finding and information.